This invention relates to moldable resin mixtures comprising polyphenylenes and graphite powder.
There has been a long felt need for polymeric compositions which can be used in extreme environmental conditions such as prolonged exposure to heat and acid. One application in which such a composition is required is in plates used in acid electrolytic fuel cells. Generally, suitable plate compositions must be electrically conductive and stable in concentrated acid solutions at temperatures about 200.degree.F. for extended periods of time. An example of a plate structure useful fuel cells is described in Dews et al. U.S. Pat. No. 3,801,374, incorporated herein by reference, in which a plate was fashioned from a graphite powder-vinylidene fluoride resin.
We have found that a graphite powder-branched polyphenylene composite formed by compression molding without solvent produces a superior plate useful in fuel cells. Such plates can withstand over 3000 hours in 100% phosphoric acid at 400.degree.F. and 0.9 volts applied electrical potential with only minimal weight loss.
In general, polyphenylenes are composed essentially of carbon and hydrogen in aromatic ring type structures, with the rings chemically linked to each other through the ortho, meta and para positions. Such polymers are to be distinguished clearly from other chemically similar phenylene type structures, such as polyphenylene oxide, polyphenylene sulfide, polyphenylene sulfone and other polymers containing the designation "phenylene". Polyphenylenes have generally been produced by techniques such as acid catalyzed oxidative coupling of the benzene ring in various aromatic compounds. The polyphenylenes produced by these processes possess some degree of high temperature thermal stability, but they are generally linear (para-polyphenylene) polymers which are relatively insoluble and infusible. Polyphenylenes have been produced which do possess certain limited solubility, but these have generally been at number average molecular weights of only about 1000 to 2000. Generally, these low molecular weight polyphenylenes contain only a low degree of branching, that is, they are still relatively linear polymers which contain long linear segments.
The branched polyphenylenes useful in producing superior graphite powder composites are those novel polyphenylenes disclosed by Wennerberg and Wang in U.S. Pat. No. 3,792,099 and produced by the process described in U.S. Pat. Nos. 3,829,518 and 3,798,281, all of these patents incorporated by reference herein. These polyphenylenes possess increased solubility over prior art polyphenylenes and excellent thermal stability over a number average molecular weight range from 1000 to over 10,000. Also, small amounts of branched nitropolyphenylene can be incorporated within the composites of our invention. Such nitropolyphenylenes are described in U.S. patent application Ser. No. 529,855 filed Dec. 5, 1974, which is incorporated by reference herein.